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UV/chlorine treatment is an efficient technology for removing organic pollutants in wastewater. Nevertheless, degradation of antihistamines in the UV/chlorine system, especially the underlying reaction mechanism, is not yet clear. In this study, the degradation of cetirizine (CTZ), a representative antihistamine, under UV/chlorine treatment was investigated. The results showed that CTZ could undergo fast degradation in the UV/chlorine system with an observed reaction rate constant (<i>k</i>_obs) of (0.19 ± 0.01) min⁻¹, which showed a first-increase and then-decrease trend with its initial concentration increased. The degradation of CTZ during the UV/chlorine treatment was attributed to direct UV irradiation (38.7%), HO^• (35.3%), Cl^• (7.3%), and ClO^• (17.1%). The <i>k</i>_obs of CTZ decreased with the increase in pH and the increase in concentrations of a representative dissolved organic matter, Suwannee River natural organic matter (SRNOM), due to their negative effects on the concentrations of reactive species generated in the UV/chlorine system. The detailed reaction pathways of HO^•, ClO^•, and Cl^• with CTZ were revealed using quantum chemical calculation. This study provided significant insights into the efficient degradation and the underlying mechanism for the removal of CTZ in the UV/chlorine system.